The present invention relates to a method and apparatus for forming a tomographic image of an object based on its absorption of a long wavelength electromagnetic field.
The absorption of electromagnetic waves has been used for over a century in the field of medical diagnosis. In particular, x-rays have been used. At the high energy and frequency of x-rays, the difference in absorption between high density tissue, such as bone, and low density tissue, such as fat, is used for contrast in the images produced.
Unfortunately, the contrast between low density materials, such as fat and water, is difficult to measure with x-rays. Furthermore, the high energy of the waves causes damaging ionization in the body.
Research in lower energy wave absorption has been limited to microwave frequencies (generally defined to be greater than 1 GHz). Attempts have been made to use microwaves to produce tomographs, computed axial tomographs, and fluoroscopic images (see U.S. Pat. Nos. 4,805,627; 4,641,659; 4,552,151; 4,271,389; 4,247,815; and 4,135,131).
The major problem with microwave image formation is that the object to be scanned is much larger than the microwave wavelength. Consequently, the conducting objects in the body act as antennae to the waves and produce multiple reflections, diffractions and refractions (see "Nonionizing Electromagnetic Wave Effects in Biological Materials and Subsystems," Johnson et al., Proceedings of IEEE, Vol. 60, No. 6, 1972, pages 692-718 and "Measurements of 1.8-2.7-GHz Microwave Attenuation in the Human Torso," Yamaura, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-25, No. 8, 1977, pages 707-710).
As a result, the decrease in intensity of the wave cannot be attributed to one source, but to many apparent sources. This can distort the image (see "Limitations of Imaging with First-Order Diffraction Tomography," Slaney et al., IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 8, 1984, pages 860-874).
In addition, because of the high proportion of the microwave energy absorbed, excessive heating and hot spots due to standing waves between conducting areas can result. If this effect is reduced by reducing the intensity or the duty-cycle, the scan time must be extended, thus increasing motion artifacts.
One approach to avoiding these problems has been the use of nuclear magnetic resonance imaging. NMR uses a large static magnetic field to induce a net magnetic moment in each voxel of the object being imaged. A magnetic field oscillating in the r.f.-range flips these magnetic moments perpendicular to the main field. The dipole moment then precesses about the main field axis at a rate proportional to the product of the gyromagnetic ratio and the main field. Spatial localization is achieved through the use of external magnetic field gradients.
Unlike x-ray imaging or r.f. absorption imaging, the precessing of the magnetic moments about the main field axis is the mechanism that provides the output signals in NMR. This mechanism provides contrast between various soft tissues and the object being imaged absorbs very little energy. Unfortunately, the generation of the main magnetic field requires the use of large and expensive electromagnets. These electromagnets represent a substantial portion of the cost and complexity of NMR systems.